This research project is directed at understanding the effect of DNA tumor virus infection on host gene expression. Studies are focused primarily on investigating the synthesis of one particular virus induced cellular enzyme, dihydrofolate reductase (DHFR). For these studies we have used methotrexate (MTX)-resistant mouse cells in which DHFR is a major gene product, accounting for as much as twenty percent of the soluble protein in some cell lines. In these cell lines DHFR synthesis is subject to control by several important, and possibly related parameters including polyoma infection, growth stimulation, and cyclic AMP.